The purpose of this work is to study the factors regulating and controlling the sudden switch from 2,3 bisphosphoglycerate (2,3-DPG) as the major organic phosphate of the chick embryo erythrocyte just before hatch to inositol pentaphosphate (IPP) as the major organic phosphate just after hatching. We isolated and purifed the enzyme 2,3-bisphosphoglycerate phosphatase-mutase from red blood cells of the day-old chick. We have also isolated and purified 3-phosphoglycerate mutase from erythrocytes of the day-old chick. The later enzyme migrates as a single protein band when electrophoresed on polyacrylamide gel containing sodium dodecyl sulfate, B-mercaptoethanol, and urea. Its estimated subunit molecular weight is 26,500 daltons. Its Km for 3-phosphoglycerate is 6.6 times 10 to the minus 4th power M and for 2,3-DPG is 8.0 times 10 to the minus 6th power M. These kinetic properties are identical to those of the phosphoglycero mutases of the erythrocytes of man.